When an object moves with respect to stationary references such as a ground plane, fixed points, lines or reference surfaces knowledge of the object's inclination with respect to these references can be used to derive a variety of its parameters of motion. In fact, inclination of the object with respect to a reference is usually required for navigating the object or obtaining information about its trajectory. Over time, many useful coordinate systems and methods have been developed to parameterize the equations motion of such objects. For a theoretical background the reader is referred to textbooks on classical mechanics such as Goldstein et al., Classical Mechanics, 3rd Edition, Addison Wesley 2002. For general examples of object tracking and inclination measurements a few examples can be found in U.S. Pat. No. 5,786,804 to Gordon and U.S. Pat. No. 6,023,291 to Kamel et al. as well as the references cited therein.
In one specific field of navigation it is important to know the inclination of an elongate object while it is in contact with a plane surface. Usually, inclination is defined to an axis of the object that passes through the point of contact with the plane surface. In some cases, this axis is also the center axis of the elongate object. Various types of elongate objects can benefit from knowledge of their inclination while in contact with a plane surface. These objects include canes such as walking canes when in touch with the ground, pointers when in touch with a display or projection surface, writing devices when in touch with a writing surface, styluses when in touch with a screen.
The need to determine inclination is deeply felt in the field of input devices such as pens and styluses. Here, inclination has to be known in order to analyze the information written or traced by the user. In principle, many methods can be adapted to measure pen inclination. Such methods can employ ranging devices using ultrasound, electromagnetic radiation including visible light and other apparatus. For example, U.S. Pat. No. 5,166,668 teaches a 3-axis detection method, U.S. Pat. No. 5,977,958 teaches a method using a difference in the time-of-flight of an electromagnetic wave and still other references teach to apply the time-of-flight method to microwaves. Still other approaches use calibration marks, e.g., as described in U.S. Pat. Appl. 2003/0025951 or entire auxiliary calibration systems as described in U.S. Pat. Appl. 2002/0141616. Still another method for measuring the inclination of a pen with respect to the vertical employs sensors mounted in the pen for measuring magnetic fields created by magnetic dipoles and oriented perpendicular to a writing board as described in U.S. Pat. Appl. 2002/0180714. Unfortunately, all of these methods are cumbersome and limiting to the user because the signals sent from the pen have to be received by external devices. In other words, the pen cannot determine its inclination independently with on-board equipment.
Clearly, it is desirable to have pen and stylus input devices that can determine their inclination independently with their own on-board equipment. In principle, pens using inertial sensors such as gyroscopes and accelerometers can be designed to derive their inclination without external devices. Japan patent application 6-67,799 proposes a method using a 2-axis acceleration sensor and the inclination angle is determined by integrating the angular velocity of the pen. Also of interest are U.S. Pat. Nos. 5,902,968; 5,981,884 using a 3-axis acceleration sensor and a 3-axis gyroscope. U.S. Pat. No. 5,434,371 teaches a structure in which an acceleration sensor is attached to the tip of a pen such to thus compensate the error due to pen inclination and a signal processing portion is located at the upper portion of the pen. Unfortunately, inertial sensors suffer from drift errors and accumulation errors that typically increase as time squared for accelerometers and linearly with time for gyroscopes.
An approach attempting to overcome the limitations of inertial sensors U.S. Pat. Appl. No. 2002/0148655 to Cho et al. teaches the use of an optical 3-dimensional detection device for detecting orientation angles of a center axis of an electronic pen relative to a ground and a height of the pen over a writing surface. Meanwhile, a 3-axis accelerometer is used for detecting movement of the pen. The optical device has a portion such as a light source for radiating a beam to the writing surface to form beam spots and a detecting portion such as a camera and corresponding optics for detecting the beam spots from the light reflected off the writing surface. This solution requires a dedicated camera and light source to detect the orientation angles and it should be noted that a significant separation between the viewpoints of the camera and the light source has to be significant in order to obtain accurate values of the orientation angles.
Still another optical method of detecting inclination in manual pen type reading devices is found in U.S. Pat. No. 5,764,611 to Watanabe. According to this method a CCD arranged in the pen type reading device picks up a dot code from the sheet and an inclination sensor arranged at the rear end of the device detects an inclined state of the device with respect to the sheet. The inclination sensor can use various mechanisms and methods for determining the inclination angle including analysis of a luminance distribution, image blurring, distance between markers on the sheet, positional shift or change in size of the markers. This solution relies on prior knowledge surface or on pre-existing data present on the surface, e.g., markers. Therefore, this method is not adaptable to a self-contained pen or stylus or other elongate object whose tip is in contact with any plane surface.